1. Field of the Invention
The present invention relates to a direct fuel injection-type spark-ignition internal combustion engine.
2. Description of the Related Art
There has heretofore been known the stratified charge combustion engine realized the directly injected fuel into a cylinder to form a mixture (hereinafter referred to as combustible mixture) that can be favorably ignited only in the vicinity of a spark plug, at the ignition timing, to burn a total lean mixture in the cylinder. To carry out stratified charge combustion, in general, the fuel injector injects fuel in the latter half period of the compression stroke. It is intended that the thus injected fuel proceeds into a concave cavity formed in the top of the piston, is vaporized by absorbing heat from the wall surfaces of the cavity, is deflected by the shape of the cavity toward the spark plug, and forms a combustible mixture near the spark plug.
In the direct fuel injection-type spark-ignition internal combustion engine for performing the stratified charge combustion, in general, the spark plug is arranged nearly at the center in the upper part of the cylinder, and the fuel injector is arranged in the periphery in the upper part of the cylinder. The piston cavity is located from nearly the central portion of the top surface of the piston to the periphery of the top surface of the piston on the side of the fuel injector. The piston cavity has a bottom wall and side walls which are spark plug side and fuel injector side. Fuel injected into the cavity collides with the bottom wall of the cavity, thereafter proceeds toward the spark plug side wall along the bottom wall, and is deflected toward the spark plug by the spark plug side wall.
In order to promote the fuel vaporization utilizing the heat of the wall surface of the cavity, the distance for moving fuel along the wall surface of the cavity must be lengthened so that the injected fuel stays in contact with the wall surface of the cavity for an extended period. In order to realize this without changing the shape of the cavity, it has been proposed to inject fuel at a relatively large acute angle with respect to the bottom wall of the cavity. This makes it possible to bring the position where fuel collides with the bottom wall away from the spark plug side wall of the cavity, maintaining the same injector hole position, compared with when the fuel is injected at a relatively small acute angle, and, hence, to extend the distance for moving fuel along the bottom wall of the cavity.
When the fuel is injected at a relatively large acute angle with respect to the bottom wall of the cavity, however, fuel which has collided with the bottom surface is diverted and all of the fuel does not proceed toward the spark plug side wall along the bottom surface; i.e., a part of the fuel proceeds toward the fuel injector side wall along the bottom surface.
Accordingly, the quantity of the combustible mixture formed near the spark plug via the spark plug side wall of the cavity is decreased, so that the engine output is also decreased. Furthermore, the mixture formed by fuel proceeded toward the fuel injector side wall of the cavity cannot be favorably burned, and unburned fuel may be emitted in increased amounts.